In the printed circuit packaging technology, an integrated circuit module is provided which includes a conventional ceramic substrate on which is mounted at least one integrated circuit chip. The integrated circuit chip has solder connections to circuitry on the ceramic substrate and the module has a plurality of conventional pins which make circuit connections to external circuitry.
A cap, which may be provided with heat sink fins, is secured to the top surface of the substrate with a solder or organic sealing material to form a hermatic seal. A suitable medium, such as thermal grease, is provided between the circuit chip and the cap to lower the thermal impedance between the chip and cap to enhance the cooling of the circuit chip during circuit operation. A circuit module of the type described is disclosed in the IBM Technical Disclosure Bulletin, Vol. 21, No. 3, August 1978, page 1064.
Suitable nozzle dispensing apparatus is used to deposit the thermal grease on the inside of the cap and in a position where it fully contacts the circuit chip when the cap is sealed to the module. Subsequent electrical testing of the module will give unsatisfactory results if the thermal grease is out of position or if the composition of the grease changes due to contamination from an unclean nozzle. As a result, the cap had to be removed and replaced by a new one which resulted in undesirable expense. Subsequently, a thermal grease was developed which consists of boron nitride and mineral oil and which could be readily dissolved by chlorinated solvents or polar organic salts. A thermal grease of this type is disclosed in U.S. Pat. No. 3,405,066. This enabled the cap to be removed, the grease dissolved, and the cap reused. However, it was found that the boron nitride and mineral oil grease had some disadvantages. This grease has a tendency to corrode if the module cap is not completely sealed tight and as a result carbon dioxide filtrates in and causes lead carbonate. Also, the composition is not stable in that the boron nitride particles tend to separate from the oil base and the grease has to be reworked to maintain its grade.
Subsequently, a thermal silicone oil base grease was developed which consists of silicone oil, zinc oxide, and fumed silica. This type of thermal grease, which is disclosed in U.S. Pat. No. 4,265,775, issued to the common assignee, solved the above-mentioned corrosion problem and proved to be safe and stable and could be stored under ambient conditions for approximately two and one half years. However, there was no known process which was capable of dissolving or stripping this type of thermal grease from a module cap so that the cap could be used again. The reason for this is that the zinc oxide particles could not be satisfactorily removed and also the silicone oil base strongly adheres to metal. It became evident that such a process was needed in order to take advantage of the superior qualities of the silicone grease.